1. Field of the Invention:
This invention relates to an image sensing device which performs exposure control by means of a shutter.
2. Description of the Prior Art:
There have been known video cameras of the kind contrived to perform exposure control by means of a shutter. The video camera of this kind, however, must have the shutter operation synchronized with a video camera scanning operation, because the camera is generally required to perform scanning at a timing synchronized with a standard television system.
For synchronizing th shutter with the camera, there has been known a method of operating the shutter at a timing determined by the logical product of a shutter release signal and a vertical synchronizing signal relative to scanning of the video camera. However, this method of the prior art presents many problems. Referring to FIG. 1 (a) of the accompanying drawings, a shutter which consists of two blades 1 and 2 is at first in repose. Under this condition, the image of an object to be photographed is prevented by the shutter blade 1 from reaching an image sensor 3. Then, as shown in FIG. 1 (b), the light from the object becomes incident on the sensor 3 as the shutter blade 1 begins to travel. The travel of the blade 1 comes to an end, as shown in FIG. 1 (c). When a predetermined period of time lapses after the start of the travel of the shutter blade 1, another blade 2 begins traveling as shown in FIG. 1 (d). Th sensor 3 then begins to be shielded from light until it is completely shielded by the blade 2, as shown in FIG. 1 (e). The operation timing of the shutter blades 1 and 2 is as follows:
Referring to FIG. 2, with a shutter release switch turned on at a point of time t1, the shutter blade 1 begins to travel at a point of time t2 synchronzed with, for example, a vertical scanning signal VD obtained immediately after operation of the switch. However, the shutter blade 1 at first has an accelerating motion and then has a constant speed motion. In order to have an exposure time accurately controlled, therefore, the light receiving surface of the image sensor 3 is preferably exposed to light after the shutter blade 1 has reached the constant speed motion. However, such as arrangement results in a delay time (t3-t2) before the light receiving surface of the image sensor 3 begins to be actually exposed to light at a point of time t3. This delay time depends on the responsivity of the shutter. In the case of a mechanical shutter, therefore, a delay time of about 8 msec is expected. Besides, it tends to vary with temperature and the lack of precision of the parts used. Furthermore, with the delay time thus being as much as 1/3 of one field period, the shutter operation would not be accomplished within the period of one field and would bring about various problems such as smearing etc. in the event of an exposure time close to one field period. The vertical synchronizing signal VD (FIG. 2) to be used for reading out must not be within a span of time from a point of time t3 to a point of tim t51. Therefore, timing for the start point t4 of the shutter blade 2 which defines the point of time t51 is important. Furthermore, since the length of time between the points of time t4 and t51 greatly varies with the degree of precision of the parts forming a mechanical shutter and ambient conditions such as temperature and humidity, precise setting of the shutter start time t4 alone does not always ensure a sufficiently adequate point of time t51 In FIG. 2, one vertical synchronizing signal VD is shown located between points of time t3 and t51. However, in this event, a smear is produced because an electrical signal representing a sensed image is read out at the timing of the vertical synchronizing signal VD.
Meanwhile, with respect to the closing movement of the shutter blade 1 the blade 1 begins to close at a point of time t4 after the lapse of a predetermined period of time Ts from the starting point of time t2 of the above-stated opening movement of the shutter. Then, the shutter blade 2 begins to actually close or shield the light receiving surface of the image sensor 3 at a point of time t5 and completely shields the light receving surface of the sensor 3 at a point of time t51.
With the device arranged in this manner, the shutter first begins to perform an opening action at a point of time t2 or according to the timing of a first vertical synchronizing signal. Then, after the lapse of an electrical delay time Ts from this point of time t2, the shutter begins to perform a closing action at the point of time t4. The time t51 after the mechanical action delay time (t51-t4) required for closing the shutter is adjusted to the timing of another vertical synchronizing signal produced for the next of electric charge transfer. However, in accordance with this arrangement, the mechanical action delay time (t51-t4) and the electrical delay time Ts are connected in series. As a result, a large error tends to develop depending on the ambient conditions such as temperature, humidity, etc. and the precision of the parts.
Theoretically, the timing of the synchronizing signal for the electric charge transfer must come immediately after the time t51 at which closing of the shutter is completed. However, in the event of the above-stated large error, some inconvenience might arise.
Furthermore, the S/N ratio decreases as the exposure time given by the shutter becomes shorter. This is because the dark current component which develops during one field accumulation period is almost constant at a constant temperature and is not relative to the exposure time.
It is a general object of this invention to provide an image sensing device which is capable of eliminating the above-stated shortcomings of the prior art.
It is a more specific object of the invention to provide an image sensing device which is capable of suppressing an adverse effect of the delayed response of the shutter.
It is another specific object of the invention to provide an image sensing device which is capable to stably adjusting the timing for shielding an image sensing means to the timing for driving the image sensing means.
It is further object of the invention to provide an image sensing device which is capable of accurately adjusting timing of the commencement of a shutter opening operation to timing of the commencement of a shutter closing operation.
It is further object of the invention to provide an image sensing device in which the S/N ratio never decreases even in the event of a short exposure time.
It is a still further object of the invention to provide an image sensing device which permits exposure time adjustment without difficulty.
It is an additional object of the invention to provide an image sensing device, wherein smears and the like do not readily arise and exposure time adjustment can be accomplished without difficulty.